DESCRIPTION (Applicant's Abstract): Control of mRNA translation is an important mechanism for the temporal and spatial regulation of gene expression that underlies the development of an organism. Translational control plays a large role in coordinating early developmental events that rely on proteins synthesized from maternally provided mRNAs. While numerous developmentally important mRNAs have been shown to be translationally regulated by sequences within their 3' untranslated regions (3UTRs), little is known about the mechanisms by regulation is achieved. Translational repression of nanos RNA plays an essential role in generating the restricted distribution of Nanos protein that is necessary for proper patterning of the anterior-posterior body axis of the Drosophila embryo. Translational repression of nanos RNA is mediated by a 90 nucleotide translational control element (TCE) within the nanos 3'UTR. The proposed work focuses on translational repression of nanos RNA in Drosophila since it affords biochemical, molecular, and genetic approaches to investigation of 3'UTR-dependent translational regulatory mechanisms. Determination of the precise step at which translation is blocked by the nanos TCE, using biochemical approaches, will provide an important framework for analysis of components of the regulatory machinery. Since TCEprotein recognition underlies TCE function, sequence and structural features of the TCE necessary for recognition as well as proteins that recognize these features will be characterized. Proteins that interact with the TCE will be identified using in vitro biochemical assays and a yeast 3-hybrid assay. The roles that these proteins play in TCE-mediated translational repression will be investigated using both biochemical and genetic assays. In a complementary approach, sensitized genetic screens will permit isolation of genes encoding components of the regulatory machinery that do not necessarily interact directly with the TCE as well as those that have more pleiotropic roles in translational regulation. These studies will lead to an understanding of how 3'UTR sequences regulate translation during development. In addition, they will shed light on how RNA-protein interactions regulate basic cellular processes to provide the highly selective control needed for development, growth, and differentiation.